I can't scrub the Movie set as image texture in cycles
Do you need the textured plane to rotoscope from? The background image works fine if you change the input button from "Image" to "Movie Clip", and deselect "Camera Clip". The movie (that I used - yours wasn't attached) worked fine for both scrubbing and playing. The plane would update OK after I stopped playing the video and would snap to match the background image. Oddly, if the properties panel in the node editor was hidden, or if another node was selected (other than the texture node), the texture stopped updating on the plane. This looks like a bug to me that you might like to report (from the menu choose Help --> report a bug). Good luck.